1. Field of the Invention
This invention relates to a headstock-reciprocating-type automatic lathe and a machining method using it, and more particularly to a workpiece supporting mechanism and method, for a headstock-receiprocating-type automatic lathe, which has first and second headstocks and is capable of performing front and rear machinings.
2. Description of the Prior Art
Heretofore, automatic lathes are known in which the gripping of a workpiece, the cutting of the workpiece in multiple steps into a desired shape by using a plurality of cutting tools, and the discharging of the finished workpiece are performed in an almost fully automated fashion. In modern machining industries, this automatic lathe constitutes an essential part of a turning center or the like and is usually numerically controlled by a computer.
Various types of automatic lathes are now available on the market; in particular, a headstock-reciprocating-type automatic lathe, in which a headstock gripping the workpiece gives to the workpiece a driving rotation and slides on the bed along the machining axis, is most widely used as a relatively small-sized and precise machining unit.
In this type of automatic lathe, since the movement of the workpiece along the machining axis is assigned to the headstock and, on the other hand, the movement of the workpiece in the cutting direction is assigned to the cutting tool, a very efficient machining can be achieved in cooperation of these two members. Further, since the headstock performs all the way from the gripping of the workpiece to the moving of the workpiece especially when many workpieces are to be taken from a continuous length of blank material in a continuous manner, it is possible to continuously machine the elongated blank material with good efficiency.
Still further, since it is unnecessary to move the cutting tool on the bed in the machining axis, a turret on which various cutting tools are detachably supported can be stably held in a proper position. Consequently, vibrations, which were inevitable with the cutting-tool-moving type, can be reduced to a minimum, thus guaranteeing excellent quality products.
However, with this type of automatic lathe, since the headstock grips only one end of the blank material during the machining, the length of a projected or overhang part of the blank material would necessarily be large if the cutting range is long. The overhang part of the blank material tends to be deformed due to lateral pressure during the machining, thus impairing the machining precision. To this end, it has been proposed to use a guide bush for supporting the overhang part of the blank material near the working point of the cutting tool in such a manner that the blank material to be machined normally projects a little beyond the guide bush to the working point of the cutting tool. This guide bush was a remedy for the bad effect of the cantilevered supporting.
This type of automatic lathe is well known as a Swiss-type automatic. Yet in recent years, it has been realized to add a second headstock to perform a rear machining, in addition to a front machining to be performed by the first headstock.
Upon completion of the front machining by the first headstock, the second headstock grips the other end of the workpiece and gives a predetermined driving rotation to the workpiece to take the final machining, thus causing an expanded function of the Swiss-type automatic lathe.
The second headstock is also slidable in the machining axis of the first headstock; as the second headstock takes the final machining and subsequently discharges the workpiece, the first headstock can stand by for the machining of the next blank material. This arrangement was accordingly very effective in shortening the entire cycle of process.
However, the primary object of this conventional double-headstock automatic lathe is to take a cutting on the end surface of the workpiece, as a rear machining, by the second headstock. This conventional lathe is used only for machining the end surface of the workpiece, which has been removed from the first headstock, to measure. That is, in the prior art, the second headstock plays as just an assistant to help the first headstock in taking the final stage of the machining of the workpiece.
Consequently, in the proportion of the front and rear machinings, the former is the overhelmingly majority. During the rear machining, all expected reduction of the machining cycle was only enough to make it ready for the front machining of the next cycle.
With this prior art, because the second headstock also supports the workpiece at only one end thereof, the allowable length of the overhang part of the workpiece is about twice or triple the diameter of the supported portion of the workpiece to secure a highly precise machining against any lateral pressure. Thus the range in which the rear machining can be performed was remarkably restricted.